Quasar disc seen around black hole

Astronomers have found an innovative way of using the Hubble telescope to study a quasar ring around a black hole.

International scientists examined the bright ring of matter, called a quasar accretion disc, possibly 300 billion kilometres wide, which is slowly disappearing into the black hole.

The team used gravitational lensing to increase the power and precision of the NASA/ESA Hubble Space Telescope and so discover the size of the glowing disc and record its colours and temperature levels. The technique is so precise that it can be compared to seeing single grains of sand on the Moon.

A gravitational lens refers to an area of matter, for instance, a cluster of galaxies, between a distant source and the observer, which is able to bend, or lens, the light from the source to the observer. It was predicted by Albert Einstein in his theory of relativity.

Although black holes are not visible, the forces they emit can create some of the brightest elements possible.

This picture shows a quasar that has been gravitationally lensed by a galaxy in the foreground, which can be seen as a faint shape around the two bright images of the quasar. Observations of one of the images show variations in colour over time. This is caused by stars within the lens galaxy passing through the path of the light from the quasar, magnifying the light from different parts of the quasar's accretion disc as they move. This has allowed a team of scientists to reconstruct the colour and temperature profile of the accretion disc with unprecedented precision. The level of detail involved is equivalent to being able to study individual grains of sand on the surface of the Moon while standing on Earth; Credit: NASA, ESA and J.A. Munoz (University of Valencia)

Quasers - or quasi-stellar objects to give them their full name - are discs of matter that glow, heat up and give off very bright radiation while orbiting black holes. They are so small that most of our knowledge of them to date is theoretical.

Lead scientist on the project, Jose Munoz, from the University of Valencia, in Spain, says, "A quasar accretion disc has a typical size of a few light-days, or around 100 billion kilometres across, but they lie billions of light-years away. This means their apparent size when viewed from Earth is so small that we will probably never have a telescope powerful enough to see their structure directly,

"This result is very relevant because it implies we are now able to obtain observational data on the structure of these systems, rather than relying on theory alone."

The astronomers used the stars of another galaxy to act as a scanning microscope and reveal the secrets of the quasar. When the stars travel across the quasar's light, the effects of gravity magnify it, providing details of its colours. Knowing the colour variation allowed them to create a full colour profile of the disc.

This diagram shows how Hubble is able to observe a quasar, a glowing disc of matter around a distant black hole, even though the black hole would ordinarily be too far away to see clearly. The diagram shows three different locations: on the left, the quasar accretion disc, which is bluer in the centre and redder around the edges; in the centre, a spot in an intermediate galaxy which the quasar's light is passing through; and on the right, Hubble's view in orbit around the Earth. The gravity from a star in the intermediate galaxy, as it passes through the beams of light from the quasar, deflects the differently coloured beams one-by-one towards Hubble. The colours seen from Hubble therefore change over time as the star effectively scans across the quasar's disc. This lets astronomers directly observe the colour, temperature and size of the disc with unprecedented precision. The precision of these observations is equivalent to seeing individual grains of sand on the surface of the Moon.; Credit: NASA & ESA

The nearer the disc gets to the black hole, the more its temperature rises and the colours become bluer, enabling the team to calculate the disc's diameter and its temperature range.

They discovered the disc is anything from 4-11 light days across (up to 300 billion kilometres). This is a very accurate figure, bearing in mind it is so far away.

Professor Munoz says, "Quasars' physical properties are not yet well understood. This new ability to obtain observational measurements is therefore opening a new window to help understand the nature of these objects."

The scientists saw minor differences in the colour of the images over time, partly due to the elements of dust in the galaxies. The light from each of the lensed images took a different journey and its colours provide information about the material in the galaxy. The researchers also found it important to record the extinction law - the journey of the dust and the calculation of how much it blocks out light.

The research, called em>A study of gravitational lens chromaticity with the Hubble Space Telescope, will be published in the Astrophysical Journal in December.

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